Opacifying pigments provide whiteness, and opacity or “hiding”, to opacifying coatings, such as paints. These pigments are present in all coatings that are designed to provide an opaque coating on and concealingly cover an undersurface or substrate surface to which the coating is applied. Opacifying pigments are absent from those coatings that are designed to be clear or transparent. Opacifying pigments are present in opacifying coatings, especially paints. In paints, the opacifying pigment is present irrespective of whether the paint is white or colored. The opacifying pigment of all paints is distinguished from the color specific pigments, also known as tinting agents or colorants, which are additionally present in colored paints. It is the color specific pigments that provide the specific color or tint to non-white paints.
It is desirable that opacifying coatings and paints have a high opacifying capacity so as to enable the coating or paint to completely conceal the undersurface, even if of a sharply contrasting color, while utilizing a minimal application of the coating or paint. It is highly desirable that complete covering of the undersurface is attained with a single application of the coating or paint, having the minimum possible thickness.
Opacifying coating and paint manufacturers have long sought to formulate opacifying coatings and paints having the desired opacity by maximizing the level of hiding for a defined level of opacifying pigment, in an attempt to approach the theoretical maximum hiding capability for a specific opacifying pigment, while minimizing the amount of opacifying pigment actually utilized.
The opacifying capacity or hiding power of an opacifying coating or paint is a measure of the coating's ability to conceal a surface to which the coating is applied. Opacifying capacity is a function of the spacing between the particles of opacifying pigment in the dried applied coating. Opacifying capacity of a coating is maximized when the light scattering capability of the opacifying pigment is maximized. Maximum light scattering efficiency occurs when the opacifying pigment particles have a certain diameter and spacing, so that the light scattering capability of each particle does not interfere with the light scattering capability of its neighboring particles. This condition may occur in coatings containing sufficiently low levels of opacifying pigment such that the individual opacifying pigment particles are isolated from each other. Coatings containing such low levels of opacifying pigment, however, do not provide sufficient whiteness and hiding at typical dried coating thicknesses. Achieving the desired levels of hiding and whiteness typically requires higher levels of opacifying pigment. At these higher levels, a statistical distribution of opacifying pigment particles occurs, which results in at least some of the opacifying pigment particles being in such close proximity to one another that there is a loss of light scattering efficiency due to crowding of the opacifying pigment particles.
Increased hiding efficiency is obtained by reducing the crowding of the opacifying pigment particles and minimizing the formation of clusters of opacifying pigment particles. One method uses polymer particles containing select chemical groups which promote adsorption to the opacifying pigment particle.
For example, U.S. Pat. No. 5,385,960 discloses an aqueous dispersion of composite particles, the composite particles each including a plurality of selected polymeric latex particles adsorbed to a titanium dioxide opacifying pigment particle. The selected polymeric latex particles have dihydrogen phosphate functional groups, which promote adsorption of the selected polymeric latex particles onto the surface of the titanium dioxide particles.
Although these composite particles provide improved hiding, there is still a need to increase the hiding efficiency provided by the opacifying pigment particles, and in particular, to obtain coatings which have hiding values at or near the maximum limit predicted by light scattering theory.
Theoretical hiding efficiency refers to the maximum level of hiding that may be obtained from a defined concentration of pigment particles and is characterized by a linear relationship between the scattering coefficient for the coating and the pigment concentration.
Titanium dioxide (TiO2) is the most common opacifying pigment utilized in opacifying coatings and paints today. Accordingly, the present invention is described hereinafter in the context of the maximum opacifying capacity for titanium dioxide, which occurs at an optimum particle diameter of from about 200 to about 280 nanometers (nm), and when the particles are spaced apart from each other at distances on the order of a few particle diameters. It is to be understood, however, that the scope of the present invention is not limited to titanium dioxide as the opacifying pigment.
Titanium dioxide is the opacifying pigment of choice of most coatings manufacturers, particularly paint manufacturers, to provide whiteness, and opacity or “hiding”, to the final dried coating. Titanium dioxide is, however, typically the most expensive raw material in a coating formulation. Heretofore, a number of techniques for minimizing the amount of TiO2, while maximizing the level of hiding provided a certain amount of TiO2 have been employed, including: (1) using titanium dioxide that has an optimal average particle size and particle size distribution for light scattering; and (2) using titanium dioxide that is well dispersed.